You assess the road conditions and conclude that the speed limit is set too low for a road that is in broad daylight and has no other cars. Are you implying that the speed limit is binarily safe, no matter the road conditions (night, rain, snow, congestion, etc)?
I doubt many drivers know their car and tyres well enough to assess the safe speed given current conditions. People either adapt to others or choose a speed they feel comfortable with. Speed limits are not perfect for every condition but humans are certainly not good at choosing a better one while driving.